Question

Consider a new instruction named branch-on-bit-set (mnemonic bbs). The instruction "bbs reg, pos, label" jumps to label if bit in position pos of register operand reg is one. A register is 32 bits wide and the bits are numbered 0 to 31, bit in position 0 being the least significant. Consider the following emulation of this instruction on a processor that does not have bbs implemented.

temp $\leftarrow$ reg and mask

Branch to label if temp is non-zero

The variable temp is a temporary register. For correct emulation, the variable mask must be generated by

• A. mask $\leftarrow$ pos
• B. mask $\leftarrow$ 0xf
• C. mask $\leftarrow$ 0x 1 << pos
• D. mask $\leftarrow$ 0x ffffff >> pos

Answer 1

The correct option is C mask $\leftarrow$ 0x 1 << pos

bbs reg, pos, label

temp $\leftarrow$ reg and mask

Branch to label if temp is non zero. If we shift one position left by pos of 0x1 then it evaluate the bbs instruction and mask $\leftarrow$ 0x1 << pos